1. Field of the Invention
The present invention relates to a plastic fiber-optic cable and a producing method for the same, and also relates to a bundle of the plastic fiber-optic cables and a producing method for the same.
2. Description of the Related Art
A plastic optical fiber (hereinafter, simply called as optical fiber sometimes) is easily produced and processed in comparison with a quartz-based optical fiber. The plastic optical fiber has a low risk for an accident in that a human body is stabbed. From these viewpoints, the plastic optical fiber is in the spotlight as an optical communication medium for a short range wherein an amount of transmission loss is left out of consideration. In particular, the plastic optical fiber is noted for home use and car use.
In general, the plastic optical fiber is constituted of a core and a clad. The core is formed from an organic compound in which polymers are in matrix. The clad is formed from another organic compound having a different refractive index (generally, lower refractive index) relative to the core. In particular, with respect to the plastic optical fiber of a graded index type comprising the core, which has graded-index distribution directing from the center toward the outside, it is possible to widen a band of an optical signal to be transmitted. Thus, this kind of the plastic optical fiber is recently noted as the optical fiber having high transmission capacity (see Japanese Patent Laid-Open Publication No. 61-130904 pp. 3–4 and International Patent Laid-Open Publication No. 93/08488 Leaflet, for instance). As one of methods for producing such a graded-index-type optical member, is proposed a certain method in which an optical member preform (hereinafter, simply called as preform) is produced by utilizing interfacial gel polymerization, and then, is stretched.
By the way, it is required that the optical fiber has small transmission loss in a high band. Moreover, it is also required that the optical fiber has moisture resistance to prevent optical properties thereof from deteriorating due to absorption of moisture. For example, a copolymer comprising cyclohexyl methacrylate and methyl methacrylate is proposed. However, in order to reduce moisture absorption, it is necessary to use a comparatively large amount of the cyclohexyl methacrylate. Thus, scope of usage is restricted. Further, as a result, heat resistance of methacrylate-based resin declines. Improving this point is required. Meanwhile, it is proposed to replace hydrogen of plastic construction with heavy hydrogen, halogen of fluorine and so forth. However, a homopolymer of monomer including fluorine sometimes causes a disadvantage regarding material stability, adhesion and so forth. This material does not have all of capabilities required for the optical fiber. By the way, the heat resistance may be improved by selecting a raw material. As an example of this, there is a polymer comprising a monomer into which an alkyl group is introduced via a carbon atom (see Japanese Patent Laid-Open Publication No. 8-220349 pp. 3–8, for instance). A glass transition temperature of this polymer, however, is not so high (see a document of “H. Kawai et al., SPIE VOL. 896 Replication and Molding of Optical Components, 69–78 (1988)” for instance) in comparison with polymethyl methacrylate (hereinafter called as PMMA). Thus, it is supposed that this polymer does not have sufficient heat resistance. Further, H of C—H bond is replaced with a large atom of Cl and Br. Such replacement is not preferable, since deterioration of reactivity is caused.
In the meantime, the optical fiber merely constituted of the core and the clad is sometimes used. However, in a case that the optical fibers are gathered in the bare state such as a bundle fiber, there arise the following problems.
i) General acrylic resin such as the PMMA has high moisture absorption so that the loss caused by the absorption of moisture increases depending on a wavelength of a light source. This phenomenon becomes remarkable, upon reducing a fiber diameter, due to a relationship between a volume of the optical fiber and a side area.
ii) It is assumed that the transmission loss increases due to damage of the clad to be caused by friction of them.
In consideration of the above, is normally used an optical member (hereinafter called as fiber-optic cable) provided with a covering layer formed on the optical fiber. When the optical fibers are attached or mounted in high density, sometimes the single-fiber plastic optical cables (hereinafter called as single-fiber cable) are bundled to be used as a multi-fiber plastic optical cable (hereinafter called as multi-fiber cable), in the purpose of saving a space. With respect to the multi-fiber cable, the following forms are known (see Japanese Patent Laid-Open Publication No. 2002-258068p.2, for instance).
iii) The plural cores having high refractive index are surrounded with the clad so as to be bundled. After that, an outer surface of the clad is covered with a protect layer (first covering layer) for protecting the optical fiber.
iv) The optical fibers having a concentric structure, in which the clad surrounds the core, are gathered to form a cylindrical shape wherein a third resin exists between the respective optical fibers. A protect layer covers the optical fibers in a mass.
v) The clad surrounds the core so as to have a concentric structure, and an outer surface thereof is covered with a first protect layer (first covering layer) to form a single-fiber cable. After that, the single-fiber cables are covered with a second protect layer (second covering layer) in a mass.
It is possible to heighten the density by reducing a diameter of the single-fiber cable and the optical fiber to be bundled.
However, it is difficult to heighten the density of the multi-fiber cables having the form of the above iii), since the covering layer of the single-fiber cable is thick. Thereupon, it is considered to thin the first covering layer in order to heighten the density of optical transmission lines. In this case, however, the multi-fiber cable has inferior moisture resistance so that a problem will be caused.